This project is designed to test the hypothesis that the developmental changes in basal and catecholamine stimulated brown adipose tissue (BAT) metabolism during fetal and neonatal life are importantly influenced by the development of thyroid function. The methods used will include the radioimmunoassay of thyroid hormone levels; in vitro measurements of tissue respiration; in vitro measurements of glycerol and free fatty acid release from BAT; measurements of tissue respiratory enzyme activities; measurements of the catecholamine stimulated adenyl cyclase-cyclic adenosinemonophosphate system; measurements of catecholamine stimulated lipase activity; and measurements of catecholamine membrane and thyroid hormone nuclear receptors. We will assess normal changes in these parameters from the fetal to the neonatal period in rabbits and rats and conduct studies in fetal and newborn animals with either thyroid hormone or propylthiouracil treatment.